A very low cost GPS receiver is highly desirable in any application which involves a mobile segment and a fixed segment in which there is a telemetry link such as a cellular telephone system or balloon radiosondes.
For instance, a cost effective method to accurately locate a Cell phone user who has made a 911 telephone call is highly desirable.
In addition, many thousands of balloon radiosondes are launched yearly in the world. Most of these are launched from commercial airports twice daily to gather meteorological data, such as data on winds aloft for flight planning purposes. Another large user of balloon radiosondes are the armed forces who need to know winds aloft in connection with artillery and missile trajectory projections. Currently, most balloon radiosondes use one of the following to determine balloon position:
Loran Transponder PA1 Omega Transponder PA1 Radar Tracking PA1 Radiotheodolite Tracking.
Both Loran and Omega are scheduled for termination within the next 20 years. Radar tracking is prohibitively expensive for most synoptic applications. Radiotheodolite tracking systems are expensive to maintain and suffer from multi-path problems at low tracking elevations. Accordingly, a need for a low cost global positioning satellite receiver has arisen.
The United States government has placed a number of satellites in orbit as part of the Global Positioning System (GPS). A GPS receiver simultaneously or sequentially receives signals from four or more satellites to determine various parameters, such as time, receiver position and velocity. Each satellite transmits two L-band signals known as L1 (1.57542 GHz) and L2 (1.2277 GHz), using a spread spectrum technique in which the carriers are bi-phase modulated with a pseudo random number (PRN) sequence or code. The L2 band transmits a code available only to authorized users and is not used in the current invention. In fact, the L1 carrier is modulated with two PRN codes, a coarse, acquisition (C/A) code and a precision (P) code and is available to any user, military or civilian. Each satellite is assigned a unique C/A and P code sequence. For the purpose of the following Disclosure, we are only interested in the C/A code modulation of the L1 carrier.
In order to determine position in three dimensions, a receiver must simultaneously or sequentially track at least four satellites. A GPS receiver is able to track a given GPS satellite when it can synchronize an internally generated replica of the C/A code with the C/A code being transmitted by the satellite. In a typical GPS receiver, the L1 signal is received by an antenna, bandpass filtered, amplified by a low noise amplifier (LNA) and then down converted to an intermediate frequency (IF) by mixing with the multiplied output of a voltage controlled oscillator (VCO). The resulting IF signal is then de-spread or correlated with an internally generated version of the satellite's C/A code sequence. A raw pseudo-range is determined by observing where in the C/A code sequence that correlation occurs at some instant in time. "Raw" refers to the determination of position prior to microprocessor compensation for clock errors, atmospheric effects, and other known factors. At least four Pseudo-ranges are processed to determine a receiver's position.
Virtually all conventional GPS receiver designs make use of a Costas Loop to decode a 50 bit per second navigation message and also use either a Costas Loop or a separate carrier tracking loop to phase lock a local oscillator to the satellite carrier and to compensate for Doppler effects.
Additionally, a codeless GPS receiver has been developed for use in balloon radiosondes as a low cost alternative to a traditional code tracking receiver. In the codeless receiver, the L1 signal is stripped of its bi-phase modulation by means of a squaring technique and then carrier Doppler information is analyzed to determine receiver velocity and position. By converting the GPS signal into two quadrature components and then multiplying the two quadrature components together, the 180_spread spectrum code is removed from the carrier frequency. The multiplied result is bandpass filtered to pass two times the expected Doppler frequency shift from which velocity information is subsequently derived.
Finally, a method is known to the art in which the wideband spread spectrum L1 signals from a plurality of satellites is frequency compressed and its Fourier components analyzed to extract velocity and position information.
The limitations of GPS technologies currently known to the art contribute to the complexity and cost of current receivers. Correlation must be performed at a down-converted frequency, requiring a local oscillator and mixer. The alternative codeless technique suffers from a signal-to-noise inefficiency which impairs the accuracy of determining position and velocity.